Inverter/ motor experts opinions?

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May 2010
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I went to a job yesterday where they wanted me to speed up a motor.

The motor is a 4KW 3 ph 1440 rpm with a gear box attached.
It at present runs direct online - with no mechanical speed control.

The machine is a (Chinese copy) pillow maker

Sheets of foam are fed into a cutting machine that cuts it into foam chunks, this is then transported on a short belt that go into a second cutter that shreds the chunks into small pieces. (Then onto the pillow filling part)

The 1st cutter and belt are on inverters to control the size of the chunks and shredded chunks.
But like always - they want more production from the machine.

If they speed up the first 2 motors (faster blades - faster conveyor belt)
The shredding motor cant cope and does not shred the foam properly.

So they want me to speed up the final shredding motor using an inverter so they can sync everything.

The motor is Chinese 380V 50HZ (currently running on 415V and has done for years)
I don't know what HZ they will want to run at (neither do they) I'd take a guess about 70HZ

What are the problems running a motor at this overspeed for a few hours at a time?
Anything I should take into consideration?


Thanks
 
More speed with same/increased load = more power required. How much power is your motor currently developing.
 
The short answer - I don't know.
It is attached to blades that shred the foam to a predetermined size dependant on the speed the foam is fed into it.
A suction fan pulls it through and on to the rest of the machine.
 
So...questions..

what speed do the inverter driven motors go at?
Are they direct driven or on gearboxes?

Is your 3rd motor on a gearbox or a pulley system that can be changed once you know the optimum speed?

What is it actually driving and what is the current being taken at the moment?
 
I went to a job yesterday where they wanted me to speed up a motor.

The motor is a 4KW 3 ph 1440 rpm with a gear box attached.
It at present runs direct online - with no mechanical speed control.

The machine is a (Chinese copy) pillow maker

Sheets of foam are fed into a cutting machine that cuts it into foam chunks, this is then transported on a short belt that go into a second cutter that shreds the chunks into small pieces. (Then onto the pillow filling part)

The 1st cutter and belt are on inverters to control the size of the chunks and shredded chunks.
But like always - they want more production from the machine.

If they speed up the first 2 motors (faster blades - faster conveyor belt)
The shredding motor cant cope and does not shred the foam properly.

So they want me to speed up the final shredding motor using an inverter so they can sync everything.

The motor is Chinese 380V 50HZ (currently running on 415V and has done for years)
I don't know what HZ they will want to run at (neither do they) I'd take a guess about 70HZ

What are the problems running a motor at this overspeed for a few hours at a time?
Anything I should take into consideration?


Thanks

I don't know much about the mechanical description you have.

Because it sounds like your shredder may have open blades that could have issues with a large increase in speed ... I'd look at the manuals to see what sort of design factor they have. Hah Hah! Since you said it was a chinese copy I expect you have no manuals at all ...

That would be my only worry about speeding up that motor. Is it possible to have more guarding added, in case the shredder fails and throws some metal around?


Just in case you want to know why I have that opinion ...

I routinely am asked to 'speed up' or 'overspeed' motors for production increases as our production guys find new bottlenecks in our process.

So far I have not had any motors fail due to the increased speed, at least directly. I have a 5 HP 575V motor driving a feed screw that has run between 90 Hz and 120 Hz for 12 years now. It has been replaced once. About average for our mill and the area that it's in if it were not on VFD.

Most of our overspeeds have been limited to 72 Hz - 120%. I don't think that is due to anything electrical. It appears to be mostly the driven load (gearbox, screw, conveyor, pump) that dictates how much things can be sped up.

Most of our oversped motors are in the 1 HP through 100 HP range, and we do the overspeed adjustment on our own, going through pump curves and gearbox specs and such. We have 6 500 HP motors that are oversped for a few minutes at a time .. but we actually got a representative from the motor company and from the pump company to sign off on that increase.
 
The issue at play is that a motor develops torque in a fashion related to the designed ratio of voltage and frequency, +-10% approximately. When you increase the frequency beyond the base design of the motor, but you don’t (can’t) increase the voltage with it to maintain that ratio, you lose torque from the motor. Whether or not your machine NEEDS that torque is a machine design consideration and in this case, as mentioned, you may have nobody to consult with on that because this machine is an apparently reverse engineered copy. So what could possibly happen is that by increasing the speed and thus DECREASING the torque, you increase the motor slip and it draws more current per unit of torque than it did, resulting in an overload condition. It might also be that the existing motor was grossly over sized to start with, so in decreasing the torque you still have more than the load requires and it works just fine. Without the original design criteria, you can only discover this by trial and error. So if the drive trips on OL, you may need a bigger motor and drive.
 
In addition to the problem with power and torque mentioned above, there is a limit on motor speed due to the centrifugal force on the rotor and the lubrication and speed limits of bearings. Typical limits are a function of frame size and the number of poles. If your motor is a Chinese POS the typicals may be irrelevant. Check with the motor manufacturer.

typical motor speed limits.JPG
 
Last edited:
My concerns,

how far is it from the vfd to the motor? load reactors may be needed. line reactors also?
a motor designed for 380 volts / 50 hz already running at 415 volts and 60 hz
is already stressing the motor and now you put an inverter on it, the motor may fail (most likely).
the motor is not rated for inverters, inverter duty motors are bigger and have a heavier design and insulation.
is the gearbox rated for the speed and torque you want?
jaref has made good points.

james
 
The issue at play is that a motor develops torque in a fashion related to the designed ratio of voltage and frequency, +-10% approximately. When you increase the frequency beyond the base design of the motor, but you don’t (can’t) increase the voltage with it to maintain that ratio, you lose torque from the motor. Whether or not your machine NEEDS that torque is a machine design consideration and in this case, as mentioned, you may have nobody to consult with on that because this machine is an apparently reverse engineered copy. So what could possibly happen is that by increasing the speed and thus DECREASING the torque, you increase the motor slip and it draws more current per unit of torque than it did, resulting in an overload condition. It might also be that the existing motor was grossly over sized to start with, so in decreasing the torque you still have more than the load requires and it works just fine. Without the original design criteria, you can only discover this by trial and error. So if the drive trips on OL, you may need a bigger motor and drive.

Disagree that this is THE issue at play. It's ONE of the issues ...

Agreed that the motor may draw more current for the same load at a higher speed.

I see this as a small change, unless the motor is already running at or above rated current. But agreed that trial and error is the only way to determine that.

If it were me doing the speed increase, I'd still be more worried about the mechanical system. But that's just me.

If the motor overloads, you put in a larger motor. If the motor rotor fails and you have a catastrophic failure of the rotor meeting the stator, it is contained inside the motor housing. At that point you may want to change the gearing to do the job without overspeeding the motor.

My apologies - I'm less about theory and more about changing stuff and observing. In my experience with documentation of a system that is not produced in the thousands ... there is never enough information to actually determine if a change will work well. Or you won't have access to it because the vendor is worried about you copying their design.

The OP asked:

> What are the problems running a motor at this overspeed for a few hours at a time?
> Anything I should take into consideration?

And I gave my opinion. It may be worth 2 cents, perhaps a bit less :D
 
All of the answers above are absolutely valid! Most importantly the safety aspects mentioned. Frying a motor is one thing, sending shrapnel through your plant is another!

There is a way to get full torque out of the motor while running it at 2X speed.
1. Wire the motor for low voltage. i.e 240 vac
2. Purchase and install a VFD rated for HIGH VOLTAGE and for the FLA of the motor wired for LOW VOLTAGE. i.e. VFD rated for 480 vac.
3. Make sure all of your wiring is done with the higher current rating.
4. Program the VFD for a max Hz that is double what the motor nameplate rating is. 120 Hz
5. Program the VFD with a custom V/Hz profile so that at 60 Hz the VFD outputs 240 vac and at 120 Hz it outputs 480 vac.

Your motor should now run all the way up to 2X speed and still produce full torque.

There is a AB publication that spells this out better than I can, plus some google searching will produce results as well. I've heard it called "super charging".

I've got a production line with 10 conveyors all hooked up this way (Done before my time and now I'm stuck with it) and it has been running for 20 years.

There are definite downsides to doing this but it is an option when your in a pinch.
 
Thanks for all the answers. I read them all carefully.
I feel a bit more confident now.
I have put a lot of inverters in without a problem but never in all this time have I put one in to permanently speed a motor above the nameplate speed.

Luckily, I have a 4KW inverter (second hand) (Control Techniques commander SK)
on the shelf.
I shall now do a temporary 'rig up' and test and observe.

I might be back - thanks
 

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